Recently fiber optic devices have been increasingly used in medical applications because of their flexibility, small diameter, low optical loss and the well developed technologies in optics. Many of them are used for surgical purposes. Due to the stringent concerns of safety especially during the laser surgical conditions in critical parts of the human body such as heart and brain, the information about fiber position and steering ability of the optical fiber tip, and others such as the result of the laser ablation of obstacles which block blood vessels are necessary. Low cost fiber-optic devices which can provide this information are still in high demand.
Many in-line fiber-optic phase compensators, modulators, switches employing piezoelectrics are currently used in optical fiber telecommunication and sensor industries. A layer of piezoelectric polyvinyledene difluoride (PVDF) polymer has been coated along the optical fiber. PVDF polymer has a few major problems; one is the low Curie temperature (85.degree. C. as compared to 300.degree. C. of PZT), low piezoelectric strain constants (e.g. d.sub.33 is 15 times smaller than that of PZT), low mechanical strength and high acoustic loss. When the temperature of a piezoelectric material is above its Curie temperature, the piezoelectricity disappears even after cooling. Low mechanical and piezoelectric strength of PVDF make this film unusable for steering an optical fiber tip. PVDF is not an efficient high frequency (&gt;10 MHz) transducer material due to its high acoustic attenation and low piezoelectricity. Since there is a large difference between the acoustic impedance, which is the product of the material density and acoustic velocity, of the PVDF and the glass, it is difficult to achieve a wide-band acoustic transducer.
Zinc oxide has been coated on optical fibers with the use of vacuum sputtering techniques. This technique is well known to be very costly, time consuming, and difficult to apply to long coating lengths especially on optical fibers. The piezoelectric strain coefficients of zinc oxide are also very low compared to those of PZT. Since zinc oxide is a crystalline structure, due to the curvature of the fiber the crystals will not be all perpendicular to the surface. Because of this tilted crystal orientation both desired longitudinal and unwanted shear acoustic waves will be excited. The signal to noise ratio for example in a broadband phase modulator is limited. Zinc oxide is a piezoelectric but not a ferroelectric material. This means that the sign of the piezoelectric strain coefficients can not be changed by external influence after the growth of the film. For bending the optical fiber it is preferred to have the freedom to change the sign of such coefficients.
Therefore, optical fiber devices equipped with mechanical flexibility and strength, miniature size, high efficiency and with low cost are still in large demand.